Undercross- Souls of the Void
by LittleTeenAuthor03
Summary: {SEQUEL TO UNDERCROSS: SUAI} Just when everything seemed to be going back to normal, just when he thought he would be free of multiverse shenanigans, Sans learned that things are NEVER what they seem. With a glitched timeline, memories of a haunting past, and ANOTHER human, our favorite bag of bones is back at it again and this time, all is not what it seems.
1. Chapter 1: Never Trust a Human

_**Author's Note:** Huzzah! Look at me! I'm back at it again with the sequel! Like, whhhaaaaaaaat? _

_I know, I'm being a weirdo. Oh well._

 _Chara: Have you noticed you're an idiot?_

 _Have you noticed you killed everyone in the Underground?_

 _Chara: *shrugs* It was fun._

 _=_= Fine, I'll just not deal with you *throws Chara into Horrortale*_

 _So, guys, I'm going to explain some things in the footnote so hang in tight! Footnote? Is that what it's called? I dunno, whateves._

 _Also, sidenote, if you guys have NOT read_ **Undercross: Strung Up about It** _then what the seagull are you doing here? Go read that before this! Well, you might not get the references but you'll probably still understand what happens in this story even without reading the last one BUT STILL! READ IT NOW MINION!_

 ** _Disclaimer: Undertale was created by Toby Fox and any other AU that appears belongs to their respectful owners. I only own the concept of Undercross (UC) and the plot, along with OCs (Original Characters)._**

* * *

 _ **Chapter 1: Never Trust a Human**_

All was going well in the Underground. Frisk was on the verge of completing yet another Pacifist Route. For all she knew, she would be able to decide what happens after saving Asriel without corrupting the Classic!Universe. It was relaxing and yet strangely nerve-racking. Of course Frisk knew what happened the last time she chose to stay with Toriel and even that went haywire.

It took less than a day for Frisk to complete her run but it always felt so much longer. She knew she saw something back in the Ruins but wasn't bothered to check it out. It took two Resets to clean out the universe's code and files but that didn't mean that all the lines of code that was previously deleted was recovered. It was like a teacup: you break it, you put it together, but if you put it together, it'll never be the same as it once was.

Throughout that entire day Frisk was in the Underground she always felt something following her. Watching her. She had an idea of who it could be but even they don't stalk Frisk like that. In fact, Chara's presence was always calming. It meant Frisk had a guide, even if she knows what would happen like the back of her hand.

The run was finally over. Frisk could finally do something new. Something that didn't involve the same thing as always. The last run was proof of that.

"Frisk…" started Toriel. Little Frisk looked up at their unofficial mother with hopeful eyes. She knew the dialogue that was to be said but she was eager to start something new. To try to stay again.

"You came from this world, right…? So you must have a place to return to, do you not? What will you do now?" Toriel asked. Frisk didn't need to think it over; the child knew what she wanted to do. She wanted to stay and try to redo her life in the previous run. She was older in the previous run, about 11, but now she was ten again. How she desired to stay in the Underground with Toriel and have Sans come on Mondays again. For the barrier to be broken, yes, but to stay in Mt. Ebott… Frisk loved it down there.

Frisk opened her mouth to speak, the two options for an answer hovering in front of her, but was cut short. Everyone was supposed to be gone, setting off to explore the Surface, so then why was Sans still staring at Frisk….?

The bag of bones was behind Toriel, distanced of course, but his sockets were blacked-out and Frisk knew what that meant. Was Sans expecting Frisk to say something else?

Yes, he was. Frisk knew it. Frisk knew that Sans didn't want the timeline to get messed up again, like the previous run, so she knew that he wanted her to choose a different option and not stay. It was simple math.

Toriel said nothing of this. The mother didn't know Sans was still there. Frisk fixed her gaze away from Sans and had full attention on Toriel now.

"I want to stay. I want to stay with you. Here. In Mt. Ebott," stated Frisk determinedly. She knew this would anger Sans, or at least disappoint him, but she didn't care. Who was he to control her? Actually, he does have a reason to and Frisk knew it all too well.

Toriel looked a little shocked but Frisk managed to still find relief in the Monster's dark red eyes. The mother smiled, a warm smile, and ruffled Frisk's brown hair softly. The child let out a giggle but her eyes landed on where Sans was _supposed_ to be, only… he wasn't…

* * *

Sans groaned as he hung up on the phone. Less than a week after Frisk completed her run and Sans was already assigned babysitter duties on Mondays. Again…

He loved Toriel, that was a fact. He would gladly do any favor for her, as any friend would, but there were times where he felt like Frisk controlled her too much. Honestly, what was the kid thinking? Mondays were the worst days of the week for Sans and Frisk knew it. They both knew that "last" Monday was a Gaster Blaster disaster and definitely made Sans, and the other versions of himself that he met, Gaster Blasters disaster casters.

Now, it was Monday again and guess what? Toriel called at the last minute ask him if he could take care of Frisk on Monday mornings. As lovable as the kid was, Sans was usually fed up with her.

 _It'll only be quick. Just the morning. Past the Ruins door. Look at you! Exploring the ONE part of the Underground that you don't know._

Sans grabbed his hoodie from the couch and started off on his journey from his house in Snowdin to the Ruins. It usually takes a good 30 minutes to an hour at most to get to the Ruins from Snowdin Town but Sans didn't complain. He would usually teleport to the Door.

In fact, he did teleport to the Door, just to save time and be lazy. Papyrus would complain; he never liked that Sans can teleport. Sans chuckled softly as he imagined his younger brother's reaction.

 _"YOU'RE SUCH A LAZYBONES!" Papyrus would yell. It never bothered Sans, it amused him. Even with so many Resets, Papyrus would always be his sunshine._

Snowdin Forest seemed colder than it usually felt. Not that Sans can actually feel the cold but something about the day seemed off. It could be one of the many missing pieces of code that Frisk couldn't restore or maybe, just maybe…. No. Sans pushed away the thought. He didn't want to deal with those other versions of himself again. Fell was a pain, Ink was a nuisance, Blue was fine, but Geno… Well, Geno was the only one Sans actually liked.

* * *

Sans knocked on the door, "Knock, knock."

"Who's there?" answered a soft but fierce voice from behind the door. It was like a mother, soft but fierce.

"Orange."

"Orange who?"

"Orange you glad I'm not late?" Sans didn't laugh this time. He used the same pun before, in the other timeline. It wasn't amusing anymore, to Sans at least. To Toriel… Well, it took her a moment to respond.

It was strange. Toriel never responded late to a pun, or any joke in general. Sans had a bad feeling about it but then again, Sans always had bad feelings about everything.

After a long moment of awkward silence, Toriel finally responded, "O-Oh, yes! Hahahahahahahaha…" It wasn't a cheerful laugh. Not good.

"Tori… you alright?" asked Sans. It took him a lot of courage to ask her that. He knew the obvious answer but he also knew the reason why.

"Hm? Oh yes, Sans. Sorry, I did not mean to worry you. Heh, heh, silly me, lost in my own thoughts. Oh! Here, let me open the door for you. It certainly is warmer in the Ruins than in Snowdin," said Toriel. The gigantic, purple Ruins Door slid open, creaking as much as stone could possibly creak.

Toriel smiled kindly, "I did not mean to call you on short notice, Sans. I, myself, had only just discovered this."

Sans nodded silently and walked in. He didn't know what Toriel was talking about but he knew it should be surprise. Whether a good surprise or a bad one was for him to see.

The first room in the Ruins was dark, with only one patch of green grass in the middle. Of course Sans knew who it was for, and God, how much he hated that flower. Even if he is Asriel, Sans still hated that flower.

The long hallway and staircase to the main room of Home was a long walk with Toriel. It was mostly silence with the occasional disturbance: "Oh, I should tell you that Frisk is doing fine. You haven't visited in a while, Sans," this and Sans that. Frankly, Sans loved the goat-mother but he was not in the mood of talking, even small talk. After the last Reset, he was more cautious and suspicious than ever before, that with keeping an eye out for that second anomaly Geno warned him about.

 _"Sans, we need to talk," stated Geno in a low voice, almost a whisper. He grabbed Sans' arm before the skeleton could leave. Geno didn't look very happy._

 _"Shoot," responded Sans in the same low volume._

 _"Chara… Keep an eye socket on that kid, Frisk I mean. I've dealt with the megalomaniac before, don't get attached to Frisk this run," warned Geno. His only usable eye was his magic eye but it didn't black out like Sans' would when he was this serious. It glowed a faint red and blue, not that bright, not the powerful…_

 _"Frisk ain't Chara, bud. I'll tell you when I don't keep an eye socket out for them," said Sans. He answered in the same threatening tone as Geno had spoken, even if more threatening. Sans almost growled._

 _Geno hesitated but then nodded, "Only warning ya, pal."_

Home looked the same as ever, not that Sans had ever seen in person but New Home was supposedly designed to mimic that of the house in the Ruins. Sans only knew that Toriel had called about a human. Who the human was had only one possible answer: Frisk. Other than that, he didn't know why Frisk hadn't already run up to him and hugged him when he entered the main room. No kid in sight, only Sans and Toriel in the living room now.

 _"Sup, Tori," greeted Sans over the phone._

 _"Oh, Sans! Hello," answered Toriel, "Sans, I need to ask you of a favor. Do you have time today?"_

 _"Yeah, Tori. What is it?"_

 _"I need you to take care of a human for me today-"_

 _Sans nearly dropped the phone. Of course Frisk wanted to try this kind of lifestyle again from the last timeline. Of course; that's just how the kid is. Sans didn't expect the Monday babysits so soon though. It had only been a couple of weeks._

 _"Sans? Sans, hello, are you there?"_

 _Sans blinked back into reality, "Huh? Oh, yeah, I'm here, Tor."_

 _"Oh well, I believe it is only going to be for today. I have to buy groceries in Snowdin so it might only take little but I still want someone to watch over them."_

 _"Yeah, yeah, sure, Tori," agreed Sans._

"Sans, could you be a dear and check the kitchen quickly? I'll check the rooms. The human does not seem to be here already," requested Toriel. She looked a little worried, probably concern for Frisk, so Sans nodded in agreement.

As promised, the skeleton moved to the kitchen. The area was tiny, only room for a handful of people, but in one glance you could see if someone was in there or not. There was no one in there. No human, only the plain kitchen.

Finding no one in the kitchen, Sans poked his head out from the doorway to see if Toriel was still busy. No in sight outside of the kitchen either.

 _Welp, might as well help myself,_ thought Sans. He popped back into the small kitchen and opened up the fridge. There they were: the row of approximately ten bottles of ketchup Toriel always saves for Sans. The skeleton took a bottle of the red condiment and started gulping it down, all while making one last check to make sure Tori didn't catch him.

Already having drunk two bottles of ketchup, Sans decided to stick around the living room while waiting for Toriel. There weren't that many rooms in Home so he didn't know why it was taking Toriel long. Frisk could be trying to play hide and seek again though. That was a possibility.

The living room was homely. The warm fire was still going in the fireplace, making Home feel cozily warm, the perfect temperature for a nap. The large chair for Toriel seemed fluffy, and the dining table was large. It was exactly like New Home but that wasn't what made Sans somewhat uneasy. No, it was what happened last timeline that did. The blue strings… The knives… The disappearances… Having to hang out with Fell… Yeah, the last one was probably the worst yet.

For some reason, there was a large window over the dining table. It had a small still under it with a flower pot on it. Inside the pot, a yellow flower had its back to Sans.

As Sans fixed his eyes on the flower, deciding whether it was just a normal buttercup, the world went dark and black. Everything dematerialized around Sans. The skeleton wasn't surprised. It was just another Reset, which meant that he had to live through yet another run. Resetting this early was unusual for Frisk. The kid often Reset after a good amount of months after the Pacifist Route was completed.

But yet, Sans _was_ surprised. He was surprised because he didn't wake up back in Snowdin. No, the world materialized again and he was in the same spot he was in Home. This confused him.

Why would a Reset glitch like this?

* * *

 _ **A/N:** So, I know that most of your guys are probably going like: "But, Soph, wait! How could you possibly make ANOTHER Undercross story? Didn't everything get closed up in the last chapter of UC?"_

 _Answer: No, not everything was filled. I left some mysteries in there along with some, uh, hints? Probably hints. Most likely hints. But yeah, hints! Basically, this second story has a new story-line with, of course, new characters and returning ones. Like, I dunno, SANS? Uh, maybe GENO? He "appeared" in a flashback so I guess, yeah, him too. And, uh, some surprises that might have not come as a surprise._

 _See ya guys in the next chapter then!_

 _DON'T FORGET TO REVIEW GUYS, I LOVE IT WHEN I KNOW PEOPLE ACTUALLY LIKE THIS (OR DISLIKE THIS)._


	2. Chapter 2: Double Trouble

_**Author's Note:** HahahahahahahahHAHHAHAHAHH ahahd io oifoNEG UHF IOkjbohfu hiwe HAHA fw bg iv iuq uu GQGQNKJQ uanfi Joi ibIQGH. __HA. HA. HA._

 _I'm sorry guys, I'm just- Let me just take a break here *deep breathe*_

 _Sorry, guys, I was just too busy laughing that I must've transferred my laughter into the computer. Sorry xD_

 _I'm not even going to explain why but it's related to this story so I'll explain on the footnote (AGAIN, WHAT ARE THEY CALLED?)_

* * *

 _ **Chapter 2: Double Trouble**_

No Reset had ever done this. It was unnatural. Then again, everything has been unnatural in the Underground lately.

Sans' first thought was that Resetting wasn't a thing anymore, that the corrupted code of the timeline had erased Resets. Then he thought that maybe the kid couldn't Reset anymore because someone else has more Determination.

Maybe.

Either way, it could have released a huge burden from Sans' shoulders. No more Resets, no more dying and living, no more different runs.

And with no more runs… Frisk was stuck in Pacifist mode until they die... And when they die… No more Frisk…

It almost made Sans giddy.

 _What?_ thought Sans. Why did he have such a thought? He hates Resets, yeah, but he still loves Frisk like family. If Frisk died, Toriel would be heartbroken. One more child to add to that list of humans that died.

One more unnecessary soul accumulated…

To distract himself from the horrible thoughts he was having, Sans tried to find something to do in Home. Toriel was most likely still checking the rooms. Or maybe her checking had been restarted and Sans didn't even know it. Or maybe… No, that wouldn't happen. If it did, Sans would've heard a scream or something. A sign.

Sans had never explored the Ruins before.

The skeleton walked out into the courtyard of Home, a strong black tree was surrounded by red rose petals at the base, and the walls were old and purple, along with the floor. Completely different from the setting in the house itself.

Sans stood in view of the black tree. He could still remember how the timeline showed signs of corruptness. There was supposed to be no tree in New Home and yet, the code glitched and mixed the values of New Home with Home, thereby adding the tree in the wrong place. The small skeleton wondered if Blue's and Frisk's values of code were also mixed up with the values of New Home. Probably the reason why they suddenly disappeared without Fell noticing anything. Frankly, blue strings were a little too outgoing in the gray background of the capital.

The mood was peaceful but Sans didn't feel at peace at all. He felt like doing… something, he didn't know what. He wanted to have a distraction. Talk with Tori, play with the kid, work on science projects even!

The room shifted slightly before going back to its original format. Sans noticed and didn't like it. Then it went black and reappeared immediately. Noticed that too.

There was the slight noise of a footstep from behind Sans and he picked up on it. He turned, in alarm, and was completely surprised by what he found.

A human, most likely to be a teenager, flinched at the sight of Sans catching him. In response, Sans had lifted the human with his blue magic, sending the human to hover some feet off the ground.

"Ah, crap!" exclaimed the human. Sans noticed that the soul was visible in color but thanks to Sans' magic, it was blue.

Deciding the human was peaceful and getting over his paranoia, Sans let go of the kid. Now details processed through the skeleton's mind clearly.

The human had white hair, completely _white hair_ , and purple eyes. He was taller than Sans, by a couple of inches, and had the strangest attire. A Delta Rune robe like Tori's and jeans with green sneakers. He also had a red scarf…

"Note to self: never try to approach a skeleton without announcing your arrival first," mumbled the human to himself while patting down his clothes.

Sans stood slightly at awe, mostly at annoyance. Of course! Tori had said _a human,_ not _Frisk._

"Who the-" started Sans.

"Hell am I?" finished the human, much to Sans' awe and confusion, "Yeah, I know. We've had this conversation before, Sans."

"How do you-"

"Know me? Yeah, again, we've had this conversation before. Not like you'll remember it but, eh," shrugged the human, "Oh, by the way, name's Damien."

"Right…" said Sans skeptically.

Damien opened his mouth to speak again but Toriel called out, "Sans? Are you out here? You know how I am with swearing!"

The goat-Monster walked out into the courtyard only to find that the skeleton she was asking for was accompanied by a human. This, however, did not shock her.

"Oh, Sans, I see you met Damien," Toriel smiled softly, standing tall like always with her hands in front of her. She always looked taller than what she really was, or was that just because Sans was relatively short?

"Yeah, sure did, Tori," Sans gave an "ok" sign and let his plastered smile seem natural. He looked over at the human, giving his magic eye glow slightly, and then turned it normal. Damien smiled nervously and shrugged.

"Hey, Toriel," he said and waved.

"Well, since you have found each other, Sans I will need to go to Snowdin so please take care of Damien," said Toriel.

"Sure can, Tor," responded Sans with a "thumbs-up."

"Then I will you two alone. I trust that you will not destroy the house?" joked Toriel before she left. Sans watched her leave done the long, purple stairs and waited for a moment to make sure that she was already in the long hallway leading to the exit of the Ruins.

Damien started to enter the house, casually, but stopped and turned to Sans, "Oh, yeah, by the way, Sans. I'm pretty sure drinking three bottles of ketchup in one go is unhealthy." He winked and left, taking a turn to the hallway with the bedrooms.

"It was two!" called out Sans, lifting up two bone fingers to emphasize.

"Tomato, tomahto!" called back Damien, waving his hand back.

Sans groaned, "Great, now there's two humans-" He stopped mid-sentence, another thought entering his mind. If Toriel was here and there was another human in the Underground, then where was Frisk? Surely Tori didn't leave the kid with _Asgore_. She despises him now, even confirming that she did not want to re-marry! So, where was Frisk?!

The first theory that came into Sans' skull was that Frisk had been kidnapped by Error again, that Sans would have to deal with even _more_ multiverse shenanigans, but the timeline seemed fine. However, there was the sudden double Reset…

* * *

"Right, how _old_ are you, kid?" asked Sans.

The skeleton was accompanied by the human, Damien, in the child bedroom of Home. The shade of red used for the walls, floor, and just about everything else in the room was not exactly what Damien would call a good combination but Sans didn't seem to mind so he didn't either.

Damien was sitting on the carpet of the room, lost in thought until Sans spoke up. It had been several minutes- maybe ten- and Damien found it an opportunity to assess the situation. He liked thinking, it gave him a peaceful feeling.

"Huh? Oh, I'm seventeen," answered Damien. He looked up at Sans, who was sitting on the bed, with distracted eyes. Sans could tell too.

"Alright, what are you thinking?"

"Uh, I-I-I just was thinking about m-my…" Damien debated about what to say. He didn't want to lie but he also didn't have to say the truth either. He was already interfering with the timeline enough and now he had to spend alone time with Classic. "…Home. I was thinking Home. This Home, not my actual home, heh."

"You had a life on the surface?"

"Uh, yes?"

"Good one?"

Damien hesitated, "Well… As good as I can live it, really." He looked down and then at Sans to see if the skeleton was still interested in talking. Of course Sans wasn't though, Damien knew that. The skeleton was _never_ much for talking; he was only doing this- taking care of Damien- for Toriel. It was all strange to the human.

Sans sighed and turned to Damien, who was still on the floor, "Alright, listen kid, I'm going to get straight to the point: Why'd ya come here? It's already obvious you're one of them multiverse guys. So: why come here? I know you're a peaceful kid, if not I would be dust by now, and I know you're hiding something, something important. So talk, or I make you talk." The skeleton blacked-out his pupils and Damien got the point immediately.

"Well…" Damien sighed in frustration, "It's a bit complicated, really. I think I just mixed this timeline with a different one. I honestly didn't mean to come here. Actually, I was trying to get to Dancetale. I ended up here and noticed the code was all messed up so I stayed to fix it and then… Well, you know what happened next."

"You were here during the fight," finished Sans, his eyes coming to realization but not much shock, "You were the second anomaly."

"And there ya go," said Damien. He pointed out his finger, snapping it into a finger-gun as if to say, "yep, that's completely right" but in more of a joking manner.

"That's out of the way then. Now: Why stay?" Sans continued with his questions. He, himself, didn't always give answers but if it is him that's giving the questions, then he likes it a lot.

Damien looked confused, "What do you mean?"

"You said you came here by mistake. You've already figured out that this not the right universe you wanted to go to. So, why _stay_?"

Damien didn't answer. He kept silent and it was getting on Sans' nerves slightly.

"Kid… I'm a comedian but I ain't _kid_ ding around here..." Sans' voice was low, so unlike him, and leaned in from the bed. Damien bit his lip, still debating on what to say. Finally, he came to a decision.

"I… I guess I'm not ready to leave yet…"

Sans kept quiet and Damien got worried. The human's been through a hard life but in recent years, Damien never thought it would be so hard to just _talk_ to Sans. He was always able to do it fine in other universes so why was it so hard with Classic? He's just like any of the other Sanses but just… _why?_

Thinking about it, Damien did feel intimidated by the skeleton, something that has never happened to him, not even in a place like Underfell or even Horrorswap… The human shivered at the thought of returning to Horrorswap again… No, just no, never again.

After several more moments of silence, Sans stood up from the bed. He strode over to the door, an alarm in Damien's head turned on. The humans jumped slightly and Sans caught the movement, giving off the plastered smile he always wore, to the kid. A wink and he was gone.

Damien stared blankly and blinked, "Wait what?" He jumped up and rushed to the door, running down the hall like if he was being chased. Down the hall and through the dining room, into the kitchen-

He should have known Sans would do this.

The kitchen was the same as ever except that Sans was raiding it again. There were only three ketchup bottles left and Sans already had one to his mouth, winking, and sending an annoyed expression to Damien.

"You scared the hell out of me."

Sans shrugged, "You're old, thought you could handle yourself."

"I thought you left me."

"Like I said." Sans took a swig of his ketchup, completely disregarding the fact that he frightened the teenage human. Then the skeleton spoke, "Ya know, kid? How the hell did you even end up like those Multiverse guys? I don't know Surface customs but I sure as damn know that kids aren't supposed to be running around universes alone."

"Frisk isn't supposed to get up and fall down Mt. Ebott alone."

"Touché." Another swig, the bottle was empty, thrown in the trashcan, another wink.

* * *

 _ **A/N:** So apparently, I've got haters now! YAY! FUN! Thing is, this guest decided to hate (not even constructive criticism, wow) and it made me laugh so much. The dude tried (emphasize on TRIED) to insult me or something but it ended up saying the opposite xD_

 _And I quote, "Such shameless promotion of your own other stories..." Now, the dude name was "Ugh" so I knew it was an attempt at hate but... It was just hilarious. Yeah, guest, I shamelessly promote my own other stories. Why? Because I am proud of them._

 _Welp, that said. Introducing DAMIEN! Yeah, he's a human. Yeah, he's strange. Yeah, Sans doesn't exactly like him xD BUT! Everything will be revealed in time.. Hush, my child, hush._

 _Also... What is Damien hiding? How did he have a conversation with Sans in that SAME TIMELINE but Sans doesn't remember it? Hmm... And what about that "Reset"? So many questions, so few answers. Ah screw it, you guys have probably already figured out Damien but... What about Sans? Why does he seem a little... off?_

 _*Fun fact: Damien is actually scared of being alone in a room. Don't ask why, it'll be answered soon ;)_

 _Alas, I am afraid I cannot say (and also because I may or may not know *wink*) BUT! With every action, comes an equal or opposite reaction!_

 _SEE YA IN THE NEXT CHAPTER, DON'T FORGET TO REVIEW!_


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